It is possible that you have switched the keyboard layout by accident to one that isn't working.
Windows remembers the keyboard layout setting per application and you may have changed the keyboard layout by accident via a keyboard shortcut.

Make sure that you have the Language bar visible on the Windows Taskbar.

You can do that via the right-click context menu of the Taskbar: Toolbars > Language Bar.

It is possible that you have switched the keyboard layout by accident to one that isn't working.
Windows remembers the keyboard layout setting per application and you may have changed the keyboard layout by accident via a keyboard shortcut.
* Make sure that you have the Language bar visible on the Windows Taskbar.
* You can do that via the right-click context menu of the Taskbar: Toolbars > Language Bar.
* http://windows.microsoft.com/en-US/windows-vista/The-Language-bar-overview - The Language bar (overview)
* Check the keyboard language (keyboard layout) setting for the application that has focus via the icon on the Language bar.
* You need to do that while Firefox has focus because Windows remembers the keyboard layout setting per application.
* The default keys to rotate the layout is a combination (Ctrl+Shift or Alt+Shift) that can easily be used in Firefox to activate a menu item.
* To avoid an unintentional switch, assign a specific key sequence (Alt/Ctrl+Shift+number) to select keyboard layouts and remove the key combination to rotate layouts (Alt+Shift or Ctrl+Shift)
* Control Panel > Regional and Language Options > Keyboards and Languages > Change keyboards > Advanced key settings > Change key sequence
See also:
* http://support.microsoft.com/kb/258824 - How to change your keyboard layout